Earthquake Probabilities for the San Francisco Bay

Region 2002-2031

Working Group 2002: Chapter 6Ved LekicEQW, April 6, 2007

BackgroundBackground

• Probabilities are weighted averages of Poisson, Brownian Passage Time, Time-predictable and Empirical probability models

• Mean probability and 95% confidence bounds

• Time period: 2002-2031• Regional and Individual Fault earthquake

probabilities

• Probabilities are weighted averages of Poisson, Brownian Passage Time, Time-predictable and Empirical probability models

• Mean probability and 95% confidence bounds

• Time period: 2002-2031• Regional and Individual Fault earthquake

probabilities

Regional Earthquake Probabilities

Regional Earthquake Probabilities

• 30 year probabilities of large earthquakes

• 30 year probabilities of large earthquakes

Regional Earthquake Probabilities

Regional Earthquake Probabilities

• Smaller quakes can be costly. 1987 M5.9 Whittier Narrows caused $350M in damage

• Historical record places bounds on probabilities of 6.0≤M<6.7 :• 1972-2001 lower bound 0.8 or Nexp = 1.6• 1850-1906 upper bound 0.99 or Nexp = 4.6

• SFBR model extended using Gutenberg-Richter with b = 0.9 predicts:• 0.96 [0.91 - 0.99] or Nexp = 3.3

• Smaller quakes can be costly. 1987 M5.9 Whittier Narrows caused $350M in damage

• Historical record places bounds on probabilities of 6.0≤M<6.7 :• 1972-2001 lower bound 0.8 or Nexp = 1.6• 1850-1906 upper bound 0.99 or Nexp = 4.6

• SFBR model extended using Gutenberg-Richter with b = 0.9 predicts:• 0.96 [0.91 - 0.99] or Nexp = 3.3

Regional Earthquake Probabilities

Regional Earthquake Probabilities

• Exposure times other than 30 years

• Similarities with Poisson Model prediction “reflect the distributed weights assigned to the “competing” models - which in turn stems from uncertainty about the effects of the” 1906 stress shadow.

• Exposure times other than 30 years

• Similarities with Poisson Model prediction “reflect the distributed weights assigned to the “competing” models - which in turn stems from uncertainty about the effects of the” 1906 stress shadow.

San Andreas FaultSan Andreas Fault

• Master fault carrying half of plate motion across the region

• 1906 had largest surface rupture of any continental strike slip earthquake

• High likelihood of floating earthquake (M6.9)

• Nearly uniform probabilities of rupture of each segment

• Loma Prieta stress change cause of SAP > SAS probabilities

• Master fault carrying half of plate motion across the region

• 1906 had largest surface rupture of any continental strike slip earthquake

• High likelihood of floating earthquake (M6.9)

• Nearly uniform probabilities of rupture of each segment

• Loma Prieta stress change cause of SAP > SAS probabilities

San Andreas Fault

San Andreas Fault

Hayward-Rodgers Creek (140 km)

Hayward-Rodgers Creek (140 km)

• Essentially two different and independent faults

• HS and HN experiencs significant aseismic creep

• Most likely to produce M≥6.7 quake• Uncertainties from:

• Depth extent of aseismic creep• Existence and position of HS-HN

segmentation point

• Essentially two different and independent faults

• HS and HN experiencs significant aseismic creep

• Most likely to produce M≥6.7 quake• Uncertainties from:

• Depth extent of aseismic creep• Existence and position of HS-HN

segmentation point

Hayward-Rodgers Creek

Hayward-Rodgers Creek

Calaveras Fault (123 km)Calaveras Fault (123 km)

• Southern two segments > 1/3 of plate motion across the SFBR and creep aseismically

• Surface breaking quake on CN between 1160 and 1425 a.c.e.

• Largest historical earthquakes in 1911 and 1984 (both M6.2)

• Segments thought to rarely link up• Uncertainties:

• Can creeping segments produce M≥6.7 since they also have high rates of moderate sized quakes?

• Southern two segments > 1/3 of plate motion across the SFBR and creep aseismically

• Surface breaking quake on CN between 1160 and 1425 a.c.e.

• Largest historical earthquakes in 1911 and 1984 (both M6.2)

• Segments thought to rarely link up• Uncertainties:

• Can creeping segments produce M≥6.7 since they also have high rates of moderate sized quakes?

Calaveras Fault

Calaveras Fault

Concord-Green Valley (56 km)

Concord-Green Valley (56 km)

• No large quakes in historical period• M5.4 on central Concord Fault in

1955• Aseismic slip present but

significance unknown source of uncertainty!

• Only M6.0 to M6.7 are likely

• No large quakes in historical period• M5.4 on central Concord Fault in

1955• Aseismic slip present but

significance unknown source of uncertainty!

• Only M6.0 to M6.7 are likely

Concord-Green Valley

Concord-Green Valley

San Gregorio (175 km)San Gregorio (175 km)

• Unlikely any activity in historical era (small probability of 1838)

• SGS under water; SGN large slip events• Multiple traces under Montery Bay• Golden Gate segmentation point uncertain• Possibility of linking SGN with SAN

neglected• Uncertain slip rate, past seismicity, effect

of 1906

• Unlikely any activity in historical era (small probability of 1838)

• SGS under water; SGN large slip events• Multiple traces under Montery Bay• Golden Gate segmentation point uncertain• Possibility of linking SGN with SAN

neglected• Uncertain slip rate, past seismicity, effect

of 1906

San Gregorio

San Gregorio

Greenville Fault (23-63 km)

Greenville Fault (23-63 km)

• Central part had M5.8 and M5.4 quakes in 1980

• Paleoseismic events of unknown magnitude occurred

• Unknown whether norther and southern segments rupture together or separately

• Central part had M5.8 and M5.4 quakes in 1980

• Paleoseismic events of unknown magnitude occurred

• Unknown whether norther and southern segments rupture together or separately

GreenvilleGreenville

Mt Diablo Thrust (20-30 km)

Mt Diablo Thrust (20-30 km)

• Blind thrust fault resulting from crustal shortening within a fold-and-thrust belt

• Treated as a single earthquake source

• Blind thrust fault resulting from crustal shortening within a fold-and-thrust belt

• Treated as a single earthquake source

Background Earthquakes

Background Earthquakes

• Faults: slip rates < 1 mm/yr; undiscovered; poorly characterized

• Significant seismicity in SFBR occurs on uncharacterized faults (Wesson 2002)

• Third-most-important source region

• Faults: slip rates < 1 mm/yr; undiscovered; poorly characterized

• Significant seismicity in SFBR occurs on uncharacterized faults (Wesson 2002)

• Third-most-important source region

Earlier StudiesEarlier Studies

• WG88 & WG90 used magnitude threshold of M≥7

• WG88: SAF and Hayward; 0.5 30-year probability for each; time-predictable probability model

• WG90: SAF, Hayward and Rodgers Creek; 0.67 30-year probability

• WG88 & WG90 used magnitude threshold of M≥7

• WG88: SAF and Hayward; 0.5 30-year probability for each; time-predictable probability model

• WG90: SAF, Hayward and Rodgers Creek; 0.67 30-year probability

Improvements in WG02Improvements in WG02• Inclusion of overall moment budget (36-43

mm/yr)• Inclusion of aseismic creep and 1989 shadow

lower probabilities on SAF and Hayward• Inclusion of background seismicity, fault

segmentation, multi-segment ruptures, and other faults

• Multiple probability models (especially BPT)• Different treatments of 1906 stress-shadow

• Inclusion of overall moment budget (36-43 mm/yr)

• Inclusion of aseismic creep and 1989 shadow lower probabilities on SAF and Hayward

• Inclusion of background seismicity, fault segmentation, multi-segment ruptures, and other faults

• Multiple probability models (especially BPT)• Different treatments of 1906 stress-shadow

Comparison of Probabilities

Comparison of Probabilities

Sensitivity of Results: 1906 shadow

Sensitivity of Results: 1906 shadow

• Uncertainty about 1906 stress change contributes to half of total uncertainty

• 2 approaches to incorporating 1906:• BPT - underestimates stress shadow; upper bound• Empirical - faults in stress shadow; lower bound

• 2 approaches neglecting 1906:• BPT without fault interactions & Poisson model

• Time-predictable model for SAF used information on slip in 1906

• Uncertainty about 1906 stress change contributes to half of total uncertainty

• 2 approaches to incorporating 1906:• BPT - underestimates stress shadow; upper bound• Empirical - faults in stress shadow; lower bound

• 2 approaches neglecting 1906:• BPT without fault interactions & Poisson model

• Time-predictable model for SAF used information on slip in 1906

Alternative PredictionsAlternative Predictions

Choice of Rupture ModelChoice of Rupture Model

• SAF and SG depend strongly on choice rupture model, while HRC and C do not

• SAF and SG depend strongly on choice rupture model, while HRC and C do not

M-logA RelationsM-logA Relations

• Determining M from A is a significant but not dominant source of uncertainty

• Determining M from A is a significant but not dominant source of uncertainty

Aseismic SlipAseismic Slip

• Used seismogenic scaling factor R:• Used to scale the

area• Used to scale the slip

rate

• Used seismogenic scaling factor R:• Used to scale the

area• Used to scale the slip

rate

AperiodicityAperiodicity

• Used in BPT• Early in cycle,

greater aperiodicity increases probabilities; late in cycle it decreases them

• Used in BPT• Early in cycle,

greater aperiodicity increases probabilities; late in cycle it decreases them